A mechanical apparatus performing a motion that imitates a human motion by using an electric or magnetic effect is called a ROBOT. The etymology of ROBOT is said to be originated in Slavic “ROBOTA (a slave machine)”. Although robots have been widely used in Japan since the end of the 1960s, most robots are industrial robots such as manipulators and transport robots used for automated production and non-man production at factories.
Stationary robots such as arm robots installed fixedly at specific sites perform operations such as a parts-assembling operation and a sorting operation only in a fixed and local working space. On the other hand, mobile robots perform operations in an unlimited working space such as acting for a prescribed or unprescribed human operation by moving flexibly along a predetermined route or without a route, and offer a variety of services that substitute for a human, a dog and other animate things. Among others, legged walking robots have advantages in moving up and down stairs and a ladder, walking over an obstacle, and a flexible walk and walking motion regardless of leveled and unleveled terrain, although these robots are more unstable and more difficult in controlling an attitude and a walk than crawler-type robots and tired mobile robots.
Recently, research and development of legged walking robots such as pet robots imitating a body mechanism and a motion of quadruped walking animals such as a cat and a dog, and human-shaped robots, i.e., humanoid robots designed by modeling after a body mechanism and a motion of biped walking animals such as a human have advanced, and thus expectations for actual use have been increasingly built up.
The following two are exemplary viewpoints from which the importance of research and development of a biped walking robot called a human-shaped robot, i.e., a humanoid robot is understood.
One is a viewpoint from human science. More particularly, fabricating a robot having a structure imitating human lower limbs and/or upper limbs and devising the control method therefor lead to technologically solving a mechanism of natural human motions including walking through a simulation process of the human motions. Such study is expected to significantly contribute to promoting a variety of other research fields for human athletic mechanisms such as ergonomics, rehabilitation technology, and sports science.
The other is a viewpoint from the development of practical robots for supporting living activities as a partner of man, that is, for supporting human activities in various daily environments including a living environment. Such kinds of robots are required to learn the way of adapting to people, each having different personalities, or to different environments while being taught by the people, and to further develop the functions thereof in various aspects of human living environments. A human-shaped robot or a robot having the same shape or the same structure with man is expected to function effectively for smooth communication with man.
For example, when teaching a robot the way of passing through a room on site while avoiding an obstacle on which the robot must not step, an operator expects to more easily teach the above-mentioned way to a biped walking robot having a similar shape with that of the operator than to a crawler-type robot or a quadruped robot having a structure totally different from that of the operator. Also, it must be the easy way for the robot to be taught (for example, refer to Takanishi: Control of Biped walking robot, Society of Automotive Engineers of Japan, Kanto Charter, <KOSO> No. 25, April 1996).
A large number of attitude control and stable walk technologies about biped-walking legged walking robots have been proposed. The stable walk mentioned above is defined as a legged locomotion without falling down.
An attitude stabilization control of a robot is extremely important for preventing the robot from falling down, because falling-down leads to suspending the performing operation of the robot and also requires a considerable amount of labor and time for standing up and restarting the operation from falling down. More importantly, falling-down of the robot causes a risk of a fatal damage to the robot itself or to an opposing obstacle colliding with the falling robot. Accordingly, the attitude stabilization control for walking and other leg-moving operations is the most important technical matter in designing and developing a legged walking robot.
While walking, a gravitational force, an inertia force, and a moment due to these forces from a walking system act on a road surface because of a gravity and an acceleration caused by a walking motion. According to a so-called D'Alambert principle, these forces and the moment balance a floor reaction force and moment as a reaction from the road surface to a walking system. As a result of the dynamic deduction, a supporting polygon formed by the grounding points of foot bottoms and the road surface has a point at which pitch-axis and roll-axis moments are zero, in other words, a (ZMP) zero moment point on the sides of or inside the supporting polygon.
Most proposals about the attitude stabilization controls and falling prevention of legged walking robots use this ZMP as a criterion for determining walking stability. Generation of a biped walking pattern based on the ZMP criterion has advantages in easily considering a toe condition of kinematic constraints in accordance with a road surface profile and the like since the grounding point of the foot bottom can be preset. Using ZMP as a criterion for determining stability means that a trajectory instead of a force is used as a target value for a motion control, thereby increasing the technical feasibility of the robot. A general idea of ZMP and an application of ZMP to a criterion for determining stability of a walking robot are described in “LEGGED LOCOMOTION ROBOTS” written by Miomir Vukobratovic (“Walking Robot and Artificial Leg” written by Ichiro Kato, et al., Business & Technology).
In general, a biped walking robot such as a humanoid robot has a center of gravity at a higher position and a narrower ZMP region for stable walking than a quadruped walking robot. Therefore, such an issue of an attitude variation depending on a road surface profile is especially important to the biped walking robot.
Some proposals using ZMP as a criterion for determining attitude stability of a biped walking robot are disclosed.
For example, a legged walking robot stated in Japanese Unexamined Patent Application Publication No. 5-305579 performs stable walking by allowing a point, on a floor at which ZMP is zero, to agree with a target value.
A legged walking robot stated in Japanese Unexamined Patent Application Publication No. 5-305581 is configured such that ZMP lies in the supporting polyhedron (polygon) or such that when landing or taking off the floor, ZMP lies in a region apart from the edges of the supporting polygon at least by a predetermined allowable distance. In the latter case, walking stability of the robot body against a disturbance increases because of the predetermined allowable distance of ZMP.
A method for controlling a walking speed of a legged walking robot by using a ZMP target point is disclosed in Japanese Unexamined Patent Application Publication No. 5-305583. More particularly, leg joints are driven for allowing ZMP to agree with the target point by using preset walking pattern data, and the rate of outputting the walking pattern data set in accordance with a detected value of an inclination of the upper body is changed. When the robot steps on an unknown irregularity and leans forward, for example, the robot regains its normal attitude by increasing the outputting rate. Also, changing the outputting rate during standing with two legs does not matter since ZMP is controlled as a target point.
A method for controlling a landing point of a legged walking robot by using a ZMP target point is disclosed in Japanese Unexamined Patent Application Publication No. 5-305585. More particularly, the legged walking robot stated here achieves stable walking such that a distance between the ZMP target point and the measured point is detected and at least one of the legs is driven so as to cancel the distance or such that a moment about the ZMP target point is detected and the legs are driven so as to cancel the moment.
A method for controlling a tilted attitude of a legged walking robot by using a ZMP target point is disclosed in Japanese Unexamined Patent Application Publication No. 5-305586. More particularly, the legged walking robot performs stable walking such that a moment about the ZMP target point is detected and the legs are driven so as to cancel the detected moment.
As described above, the essence of the attitude stability control of the robots which use ZMP as a stability determination criterion is to compute a point, at which a pitch-axis moment and a roll-axis moment are zero, on the sides of or inside a supporting polygon formed by grounding points of foot bottoms and a road surface.
However, a prior study conducted by the inventors, et al. reveals that when a robot performs a legged motion at high speed, a moment about yaw axis, i.e., about Z-axis is generated on the robot body in addition to pitch-axis and roll-axis moments.
FIG. 11 illustrates the exemplary relationship between a walking speed (s/step) and a moment generated about a yaw axis (Nm) of a biped walking robot. The drawing indicates that the shorter the time for one stride of the legged walking robot, that is, the higher the walking speed, the more significantly the yaw-axis moment increases.
Such a yaw-axis moment acts to rotate the robot body in the course of time, thereby causing a slip about yaw axis between the foot bottom of the robot and the road surface. This slip becomes an obstacle such as affecting the walking stability for stably and accurately achieving an expected leg-moving operation. Furthermore, an extremely significant affect of this yaw-axis moment may cause the robot to fall down and accordingly may involve damage of the robot body or the obstacle colliding with the robot.
For example, disclosed in the specification of Japanese Patent Application No. 2000-206531 which has been already assigned to the same assignee is a method for producing a whole-body motion pattern, in which a stable walk of a biped walking robot is achieved, by deriving a waist motion pattern based on an unprescribed foot motion pattern, ZMP trajectory, trunk motion pattern, and upper limb motion pattern. According to a walk controller and a walk control method of the robot stated in this specification, the locomotion of the lower limbs for stable walking is determined even when the robot lies in any of variously different states such as a standing-straight-and-stiff state and a normal walking state. In particular, when a body gesture motion and a hand waving motion are applied in a standing-straight-and stiff state, the locomotion of the lower limbs for stable walking is determined in response to such locomotion of the upper body.
However, although a method for deriving a whole-body coordinated motion for stable walking by canceling roll-axis and pitch-axis moments Mx and My generated on the robot body at the preset ZMP by motions of the feet, the trunk, and the upper limbs of the robot is stated in this specification, a yaw-axis moment Mz generated during the whole-body coordinated motion is not considered.
To overcome the above problem, a method is also considered in which after a slip about the yaw axis of the robot body is detected by a sensor or the like, a compensation control for canceling the yaw-axis moment is performed by swinging arms and the like. However, the compensation control is performed in an ex-post manner in this case, thereby always giving rise to a problem of a more or less degree of slippage.
Also, since a motion of swinging arms and the like is a motion in a plane irrelevant of a force of gravity, a control for placing the arms into desired positions is needed after the moment is canceled.
Moreover, human-shaped arms configured by rotational joints having a common motion in the Z-axis direction are likely to cause pitch-axis and roll-axis moments which damage the stability of the robot when the arms are swung for canceling the yaw-axis moment.
That is to say, swinging the arms for suppressing the yaw-axis slippage in an ex-post manner gives rise to a problem of causing unstable walking while overcoming the problem of the slippage about the yaw axis.
Since this phenomenon is more significant especially when the robot performs a motion at a higher speed, the above control method is not suitable for a robot moving (running) at a high speed.
In addition, generating only a yaw-axis moment by using human-shaped arms is likely to produce a non-human and unnatural motion (for example, a motion turning a barbell in a horizontal plane so as not to generate a motion in the Z-axis direction), accordingly this problem is crucial for an entertainment robot putting importance on power of expression.